Omnidirectional dipole loop antenna array

ABSTRACT

An omni-directional aerial, more especially for the reception of domestic television programs in the U.H.F. band, comprises four folded dipole elements (1-4), each formed as a self-supporting open-ended wire loop (1A, 1B, 1C, 1D) integral with twin feeder portions (1E, 1F; 2E, 2F; 3E, 3F; 4E, 4F). The loops are arranged in planes parallel to a common central axis and are regularly angularly displaced about this axis so that opposed pairs of loops lie in parallel planes. The twin feeder portions (1E, 1F; 2E,2F; 3E,3F; 4E,4F) are connected in common to respective terminal means at the central axis, and each pair of feeder portions of each loop extend parallel to one another in a plane radial to the central axis which enables simple matching of the impedance of the aerial to a connecting cable.

This invention concerns improvements in and relating to aerials, andmore especially to aerials for radio and television reception in the VHFand UHF frequency band.

Conventional dipole aerials used for the reception of radio andtelevision in the VHF and UHF frequency band have directionalcharacteristics such that for optimum gain they must be aligned in apredetermined position relatively to the transmitter. Such aerialstherefore have significant disadvantages when used on mobile vehicles,since, with the aerial in the fixed position on the vehicle, thestrength of any signal received will vary in accordance with thedirection in which the vehicle is moving and the corresponding alignmentof the aerial relatively to the transmitter.

Attempts to produce omni-directional aerials for use on mobile vehicleshave hitherto not proved entirely successful.

It is accordingly an object of the invention to provide a radio ortelevision aerial of which the construction is such that acceptablesignal reception can be obtained regardless of the relative alignment ofthe aerial and the transmitter, at least in the horizontal plane.

An aerial construction is known, (see U.S. Pat. No. 4,479,127) whichcomprises four substantially identical folded dipole elements eachelement defining a loop lying in a plane parallel to a common centralaxis which, in use, is intended to be vertical, the planes of all ofsaid loops being equally spaced from said central axis and the planes ofadjacent ones of said loops being relatively angularly displaced by 90°about said central axis such that said dipole elements are arranged inmutually facing pairs disposed on opposite sides of said central axis,all of said dipole elements being connected to common whereby the polarresponse of said aerial is substantially circular about said axis. Suchan aerial, in the case of which a horizontally polarisedomni-directional radiation pattern is obtained over a wide bandwidthwhen used as a transmitting aerial, is, however, of relativelycomplicated construction, special means being necessary to obtain therequired impedance of the aerial over the appropriate bandwidth. Thisaerial does not therefore meet the need for an omni-directionalreceiving aerial of simple and robust construction suitable for use bythe consumer for the purpose of reception of domestic radio ortelevision programmes.

In accordance with the invention, however, this object is achieved by aconstruction as outlined above which is characterised in that eachfolded dipole element consists of a single open ended loop of which theends are spaced apart in the respective plane in a direction parallel tothe said central axis, and that the respective ends of each loop arecoupled to common terminal means, by means of twin feeders extendingfrom said ends of the loop parallel to one another and radially towardssaid central axis.

The size and shape of each dipole element may vary in accordance withthe frequency of electromagnetic waves to be received, and in the caseof a UHF television may comprise loop aerial elements of round or squareconfiguration. Square elements are found to be more advantageous,particularly when opposite corners of the square are aligned on verticaland horizontal axes respectively, since the aerial can be used for thereception of both horizontally and vertically polarised transmissions.

In the case of an aerial for reception of VHF frequencies, which areusually horizontally polarised, the dipole elements are preferably inthe form of folded dipole of which the horizontal dimension is greaterthan the vertical.

An aerial in accordance with the invention has surprisingly been foundto give a response that is truly omni-directional, whilst also having avery wide band-width.

One embodiment of aerial in accordance with the invention is illustratedin the accompanying drawings, in which

FIG. 1 is a perspective view of an aerial in accordance with theinvention.

FIG. 2 is a view similar to FIG. 1 with components of the aerialassembly removed to show more clearly the configuration of individualaerial elements, and

FIG. 3 is a polar diagram illustrating the response of the aerialillustrated in FIGS. 1 and 2.

Referring to FIGS. 1 and 2 of the drawings, there is shown an aerial inaccordance with the invention which is intended for use in the receptionof United Kingdom standard television programmes in the UHF band. Itwill be appreciated that such an aerial must be capable of receivingtransmissions with the electrical field polarised in either thehorizontal or the vertical plane. Its beam width in the vertical planeshould be narrow so as to reduce ground, thermal and man made noise. Itmust present a reasonable 75 ohm impedance at all frequencies in therange so as to preserve teletext eye height, prevent picture ringingeffects and/or not degrade the noise performance of any associatedaerial amplifier. The aerial comprises four individual dipole aerialelements 1 to 4, each of which comprises four rectilinear sectionsillustrated in FIG. 2 at 1A, 1B, 1C and 1D in the case of the element 1,which sections are arranged in a common plane to form an approximatelysquare loop aerial. The respective aerial elements are supported from astand 5 by stem portions, illustrated at 1E and 1F in the case of theelement 1, the stem portions extending at right angles to the plane ofthe square loop. Corresponding portions of the remaining aerial elementsare indicated by like reference letters and will not be referred toindividually, all such elements being identical in construction.

As illustrated, each square loop element is located in a vertical planewith diagonally opposite corners lying on a horizontal axis, the planesof the oppositely disposed elements being mutually parallel.

The stand 5 comprises a pedestal portion 5A terminating in a horizontalplatform 5B containing a central recess 5C bounded by a shallow uprightcylindrical wall 5D. The wall 5D contains four openings arrangedcrosswise to receive the lower stem portions 1F, 2F, 3F and 4F of theaerial elements. The upper stem portions 1E, 2E, 3E and 4E of the aerialelements are located in spaced relation to the lower stem portions bymeans of a spacer member 6 which comprises a central cyclindricalportion 6A and four integral webs 6B which extend crosswise to liebetween the respective stem portions of each aerial element. The freeends of the stem portions 1E, 2E, 3E and 4E are received in grooves, notshown, in a top end wall of the central cylindrical portion 6A and areclamped in place by means of an electrically conductive end plate 7secured to the member 6 by means of self-tapping screws 8. The end plate7 serves to provide an electrical contact between the central conductor9 of a co-axial aerial cable, the spacer 6 being of electricallyinsulating material so the upper and lower stems portions of therespective dipole elements are electrically isolated from one another.The lower stem portions 1F,2F,3F and 4F are likewise electricallyconnected together by means of a corresponding end plate, not shown,secured to the underside of the spacer 6 and coupled to the outerscreening conductor of the aerial cable.

As shown in FIG. 1, the spacer member 6 and the aerial elements clampedthereto are secured in place on the stem 5A by means of a shroud member10 of moulded electrically insulating synthetic plastics material whichis arranged to make snap engagement with the platform 5B in a manner notshown.

The horizontal corners of the respective aerial elements are linked bymeans of insulator members 11 moulded in halves from electricallyinsulating synthetic plastics material, and assembled by means ofself-tapping screws 12.

It will be seen from the above disclosure that the preferred embodimentof the invention provides a convenient and compact aerial assembly thatis of elegant appearance. Moreover, the assembly illustrated has beenfound to have a surprisingly good 360° signal acceptance in thehorizontal plane, the horizontal polar diagram being almost perfectlycircular over a wide range of frequencies, as shown in FIG. 3.

It will be appreciated that various alterations and modifications may bemade to the arrangement described, without departing from the scope ofthe invention. By way of example, however, in the specific embodimentillustrated, the square dipole elements 1 to 4 were formed of wire 4.8mm in diameter and shaped to define a square loop of approximately 12 cmalong each side, whilst allowing a distance between the centres of theperpendicular stem portions of approximately 3 cm. The latter form twinparallel feeders having a 300 ohm impedance over the bandwidth of theaerial and thus when the four feeders are connected in parallel theimpedance of the aerial matches the 75 ohm impedance of the standardcoaxial cable used for television aerial connection. The distancebetween the oppositely disposed dipole elements was approximately 19 cmcorresponding to half the wavelength of the central frequency of the UHFband and the resulting assembly was found to have a usable bandwidthextending over the full UHF television frequency range of 470 to 860MHz. Thus a convenient and compact UHF television aerial was providedsuitable for use on any mobile vehicle such as coaches, cars, yachts,ships etc.

We claim:
 1. An aerial comprising four substantially identical foldeddipole elements each element defining a free-standing loop lying in aplane parallel to a common central axis which, in use, is intended to bevertical, the planes of all of said loops being equally spaced from saidcentral axis and the planes of adjacent ones of said loops beingrelatively angularly displaced by 90° about said central axis such thatsaid dipole elements are arranged in mutually facing pairs disposed onopposite sides of said central axis, all of said dipole elements beingconnected in common whereby the polar response of said aerial issubstantially circular about said axis, characterised in that eachfolded dipole element consists of a single open ended loop of which theends are spaced apart in the respective plane in a direction parallel tothe said central axis, and that the respective ends of each loop arecoupled to common terminal means by means of twin feeders extending fromsaid ends of the loop parallel to one another and radially towards saidcentral axis.
 2. An aerial as claimed in claim 1 characterised in thatthe loops defined by said dipole elements are of quadrilateralconfiguration, with one diagonal of each quadrilateral lyingapproximately in a plane at right angles to said axis.
 3. An aerial asclaimed in claim 2 characterised in that it is a television receivingaerial and that the loops defined by said dipole elements are ofgenerally square configuration.
 4. An aerial as claimed in any one ofclaims 1-3, characterised in that the loop of each dipole element isformed integrally with said twin feeders as a self-supporting wirestructure, and that each dipole element is supported by mounting meansengaging said twin feeder portions.
 5. An aerial as claimed in claim 4,characterised in that said dipole elements are further mechanicallylinked by electrically insulating means connecting the nearest portionsof each adjacent pair of loops, whereby the loops are linked together toform a ring.
 6. An aerial as claimed in claim 4, characterized in thatsaid mounting means comprises a member of electrically insulatingmaterial, said member comprising a hub portion concentric with saidcentral axis and four web portions extending radially outward from saidhub, the twin feeders of each dipole element being received incorresponding radial grooves or recesses of said hub and verticallyspaced apart by a corresponding one of said web portions.
 7. An aerialas claimed in claim, 6, 10, 11 or 12 characterised in that said twinfeeders are clamped in place in said grooves by electrically conductiveplates secured to axial ends of said hub portion and forming saidterminal means.
 8. An aerial as claimed in claim 7, characterised inthat said hub portion is located upon a stand having a central stem forreceiving a coaxial cable to be connected to said terminal means.
 9. Anaerial as claimed in claim 8, characterized in that said hub portion issecured upon said stand by means of a shroud of electrically insulatingmaterial which is shaped to enclose said hub portion, said webs and thecorresponding portions of said feeders and is secured to a supportingplatform of said stand.
 10. An aerial as claimed in claim 5,characterized in that said mounting means comprises a member ofelectrically insulating material, said material comprising a hub portionconcentric with said central axis and four web portions extendingradially outward from said hub, the twin feeders of each dipole elementbeing received in corresponding radial grooves or recesses of said huband vertically spaced apart by a corresponding one of said web portions.11. An aerial as claimed in claim 4, characterized in that said mountingmeans comprises a member of electrically insulating material, saidmember comprising a hub portion concentric with said central axis andfour web portions extending radially outward from said hub, the twinfeeders of each dipole element being received in corresponding radialgrooves or recesses of said hub and spaced apart by a corresponding oneof said web portions.
 12. An aerial as claimed in claim 5, characrerizedin that said mounting means comprises a member of electricallyinsulating material, said member comprising a hub portion concentricwith said central axis and four web portions extending radially outwardfrom said hub, the twin feeders of each dipole element being received incorresponding radial grooves or recesses of said hub and verticallyspaced apart by a corresponding one of said web portions.